c.m4: Use __has_attribute to check for attribute availability.

[Jacek Caban]

This fixes -Wunused-but-set-variable warnings on clang msvc targets. Commit
a166d5450e3f47587b98f6005f9f5627dbe21a5b fixed those warnings in most 
cases,
but currently YY_ATTRIBUTE_UNUSED is defined only when __GNUC__ is defined.
When using clang in msvc mode, it's not defined, but __unused__ attribute
is still available.


In my use case, it fixes warnings when building the Wine project with 
clang as

a cross compiler.

0001-c.m4-Use-__has_attribute-to-check-for-attribute-availa.txt
Description: Text document